1. Field of the Invention
The present invention relates to photolithography applied for precise processing when semiconductor devices are produced, and more particularly relates to an optical exposure method in photolithography.
When semiconductor devices such as ultra-LSIs are highly integrated and precise processing is required, manufactures greatly rely on improvements in lithographic technology. Photolithography using light is suitable for mass production. Therefore, it is adopted for the economical reasons.
2. Description of the Related Art
In order to improve resolution in optical exposure technology, it is important to increase the numerical aperture (NA) and to reduce the wavelength of light generated by a light source. On the other hand, the focal depth is reduced as NA is increased. Recently, a deformation illumination method (an oblique incident illumination method) improving the critical resolution and focal depth has been given attention (for example, shown in pages 28-37 of "Nikkei Micro Device" No. 82, April, 1992).
As holes in the diaphragm (the apertures) in the deformation illumination method, zonal holes and four holes symmetrical with respect to a point are well known. In a conventional illumination method, a ray of illumination light sent from a circular hole coinciding with an optical axis to a photomask (reticule) is vertically incident and an image is formed by three beams of light of 0, +1, and -1. However, in the deformation illumination method, the position of the diaphragm is shifted from the optical axis, so that illumination light sent from the hole is obliquely incident on the photomask, and image formation is conducted by two beams of light of 0 and +1 sent from the photomask. In a focal position, higher contrast can be provided by the conventional illumination method, however, in a defocal position, higher contrast can be provided by the deformation illumination method, so that the focal depth and resolution can be considerably improved.
In the conventional deformation illumination method, i.e., only for a simple line and space pattern, a pattern of the photomask is projected and exposed on a register with a diaphragm having the aforementioned general type of diaphragm holes. Accordingly, the illumination system does not meet the requirement of each pattern, so that the effect of oblique incidence of the deformation illumination method is not sufficient.
Also, recently, a lithographic technology using a phase shift mask has been given attention, and the following pattern forming method has been reported to be an effective technology: an unexposed portion (pattern) is used that is accompanied by a sharp decrease of optical intensity generated by a step portion (the phase of exposure light is changed by 180.degree. by this step portion) of a phase shifter of a phase shift mask.
However, when a pattern is formed by this technology, the unexposed portion (pattern) is formed in all step portions of the phase shifter. Therefore, in many applicable fields, it is necessary to provide a process to inhibit the formation Of a pattern generated by the unexposed portion generated by an unnecessary step portion of the phase shifter.
Therefore, the following techniques have been conventionally proposed to ease the sharp decrease of optical intensity: another exposure mask is put on the unnecessary unexposed portion so as to conduct an exposure operation (double exposure); and a multi-shifter (step of 90.degree.) is provided stepwise in a step portion of the phase shifter, the pattern formation of which is not necessary.
However, in the double exposure method that has been conventionally proposed as a method to remove an unnecessary unexposed portion, it is necessary to manufacture a plurality of masks so as to conduct multi-exposure Accordingly, it is necessary to increase the number of the mask manufacturing processes. On the other hand, it is also necessary to ensure an alignment of double exposure, so that the throughput is lowered.
Moreover, when a multi-shifter is manufactured, complicated and difficult process technique is required in order to provide an optically accurate multi-shifter, and further a big problem is caused when a manufactured phase shift is inspected and corrected.
In order to meet the demand of forming minute patterns, for example, attention is given to an oblique incidence illumination method disclosed in the official gazette of Japanese Unexamined Patent Publication No. 2-142111 (1990). According to this method, a ray of light that is vertically incident on a lens is incident being oblique at a predetermined angle, so that focusing is conducted using interference of light.
However, in the aforementioned conventional method, the same light source is used for any device patterns without giving attention to the profile of the light source. Accordingly, the following problems are caused.